Dengue and Ebola are both enveloped viruses that enter cells by membrane fusion. Aspects of the structures of their fusion proteins are known. Blocking membrane fusion by a viral envelope protein can prevent entry and infectivity, as shown by peptides such as T-20, an HIV antiviral now in clinical trials. In collaboration with the chemical screening core facility of the NERCE, we propose to develop structure-based screens for small molecules that will inhibit the Ebola and dengue fusion steps. In the case of Ebola, which has a Class 1 fusion protein, the strategy will follow one we have used successfully to discover small molecules that block envelope-dependent HIV fusion. In the case of dengue, which has a Class 2 fusion protein, our recently determined structure of the dengue envelope protein suggests a binding site for potential inhibitors; we will use this information to design our screen. Understanding the nature of the conformational change that leads to fusion is likely to provide additional avenues for structure-based inhibitor discovery. We therefore propose to determine the structure of the fusion-promoting ("low pH") conformation of the dengue envelope protein. To augment further our picture of flavivirus envelope structure, we propose to determine the arrangement of proteins on the surface of immature virions by three-dimensional image reconstruction from electron cryomicroscopy of inactivated, immature particles of tick-borne encephalitis virus.